According to contemporary forms of shearing handpieces crank drive to a cutter reciprocating over a fixed comb is derived through a fork supported upon a medial pivot and to a rear end of which drive is applied. The cutter is secured by so-called "chicken's feet" to the forward end of the fork and closing pressure between the cutter and the comb is determined by an adjustable thrust pin (traditionally called a "tension pin") bearing upon the fork's forward end. Such an arrangement is disclosed in Australian Patent Specifications Nos. 7900/07 and 12099/28. However, it is known that uneven wear between the cutter and the comb arises due to a twisting force imposed upon the fork as the cutter reciprocates towards at least one of its extreme lateral positions with respect to the comb. Inefficient shearing also results.
Attempts to overcome this problem have been made, one of which is disclosed in Australian Patent Specification No. 551 623, where an abutment is provided to engage an intermediate portion of the thrust pin to impart bending movement thereto to restrain rocking motion of the crank drive. A further proposal is disclosed in U.S. Pat. Specification No. 4 291 462 where the conventional thrust pin is replaced by a bent tensioning arm which is pivotally mounted with one end bearing upon the fork and its other end extending on an axis colinear with the pivotal axis of the fork. These proposals have only met with limited success mainly due to the rigorous task to which these appliances are applied. The invention of U.S. patent application Serial No. 809 503, having a common assignee with the instant application, is a recent proposal for improvement.